Regarding the molding methods for producing molded components of various molded articles using a molding source material made of ceramic powder, metal powder, or a mixture powder of these two powders, there are various molding methods such as cast-molding, press-molding, and injection molding. Basically, these molding methods are production methods in which a ceramic powder, a metal powder, or a mixture powder of these two powders is put into a forming mold in a dispersed state and cured in the forming mold to form a molded article, and the formed molded article is released to obtain a molded article. In the production methods, suitable molding methods are selected in accordance with the form of the molding source material to be used, the shape of the molded article to be produced, and the degree of complexity of the structure.
In the meantime, the produced molded articles are greatly diversified, varying from those having a simple shape or structure to those having a complex shape or structure, from those having a small thickness to those having a large thickness, and from those having a high dimension precision to those having a low dimension precision with regard to these, depending on the required properties of the molded components. In the case of producing these molded components, whichever molding method is adopted, it is important that the molded articles can be formed with good dimension precision in a forming mold and that the formed molded articles can be released easily without any damage from within the forming mold. In order to deal with these matters, with a premise to adopt an accurate and appropriate molding method, it is required to give full consideration to the molding source materials and forming molds. Further, if the molded components have a special shape or structure, these considerations will be all the more important.
If the molded components have a complex shape or structure, depending on the degree of complexity in the shape or structure, there are problems such as the difficulty of forming the molded articles in an ordinary mold with the use of an ordinary molding source material, the difficulty of releasing the molded articles from within the forming mold, the decrease in the dimension precision of the shape or structure, the difficulty of densifying the molded articles, and the possibility of generating cracks in the case of the molded articles having a large thickness.
As one means to deal with these problems, it is known to adopt the gel-cast method which is one cast-molding method. In Tokkai No. 2001-335371 which is a Japanese Laid-Open Patent Publication, a gel-cast method giving consideration to slurries for molding constituting the molding source materials is disclosed.
The gel-cast method makes use of a source material powder made of ceramic powder, metal powder, or a mixture powder of these two powders as well as a dispersion medium and a slurry containing a gellifying agent, as a molding source material. It is a method in which, after the slurry is injected into a forming mold, the slurry is held at a predetermined temperature in the presence of a cross-linking agent for curing to form a molded article. Since the gel-cast method involves injection of a before-cure slurry in a highly fluid state into the forming mold, the gel-cast method has great advantages such as the facility of forming molded articles having complex shapes or structures and the property that the formed molded articles have sufficient strength to withstand the handling by curing of the slurry.
In a conventional gel-cast method, the slurry to be used is prepared by the addition and dispersion of a source material powder and a gellifying agent containing a prepolymer such as polyvinyl alcohol, epoxy resin, or phenolic resin as a major component, into a dispersion medium. The slurry prepared in such a manner is injected into a forming mold made of metal or the like for curing. Here, a cross-linking agent is added to the slurry injected into the forming mold, whereby the slurry is cured to form a molded article by cross-linking reaction between the added cross-linking agent and the gellifying agent.
In the meantime, in the conventional gel-cast method, since the prepolymer constituting the gellifying agent is present in a diluted state in the non-reactive dispersion medium, it is necessary to use a large amount of prepolymer and cross-linking agent in order to attain full curing of the slurry. If this means is adopted, the viscosity of the slurry will be high, thereby deteriorating the fluidity of the slurry.
As a result of this, it is difficult to disperse both of the gellifying agent and the cross-linking agent uniformly in the slurry. This makes it impossible to cure the whole slurry uniformly and increases a possibility that cracks may be generated in the formed molded article at the time of drying or baking. Further, the workability requested in the work of injecting the slurry into the forming mold is not sufficient. In addition, it is difficult to make the slurry extend uniformly into the forming, so that the density of the formed molded article will not be as intended, and the molded article will have a large shrinkage at the time of drying or at the time of baking. This increases a possibility that cracks may be generated at the time of drying or baking.
Further, since the slurry contains a large amount of non-reactive dispersion medium, prepolymer, and cross-linking agent, the molded article formed with the use of the slurry as a molding source material contains a large amount of components that are evaporated and/or lost by burning. For this reason, a large shrinkage occurs in the formed molded article at the time of drying and baking. This will be a large factor in causing deformation of the molded article, generation of cracks, and poor sintering.
Furthermore, since the conventional gel-cast method adopts a forming mold made of metal as a forming mold and when the formed molded article is released from the forming mold, there is a possibility that, particularly in the case of producing a molded article having a fine shape or structure or having a complex shape or structure, the portions with the fine or complex shape or structure of the molded article to be released may be damaged at the time of releasing. In addition, if the forming mold has a portion with an undercut shape or structure, the molded article formed within the forming mold cannot be released without being damaged.
On the other hand, as a molded article having a special shape or structure and used for special purposes, a hollow ceramic molded article having a hollow body part and a narrow pipe part can be mentioned. The sintered body formed by baking the hollow ceramic molded article is used as a hollow ceramic component that have various functional components such as a discharge vessel for a high-pressure electric discharging lamp. Generally, in the hollow ceramic molded article which is a precursor of the hollow ceramic component such as a discharge vessel, the body part and the narrow pipe part are formed as separate parts, and these two are joined with each other and assembled to construct the hollow ceramic molded article.
In recent years, in order to improve the function of the hollow ceramic components and to eliminate the cumbersome work of assembling the construction members in forming the hollow ceramic molded articles, an integral-type hollow ceramic molded article integrally having a body part and a narrow pipe part as well as a hollow ceramic component formed from a precursor made of the ceramic molded article are proposed in International Patent Application No. PCT/JP99/02777 (International Publication No. WO99/62103).
In the hollow ceramic component, from the viewpoint of functions, it is demanded that the inner circumferential surfaces of the body part and the narrow pipe part as well as the boundary parts of these are as smooth as possible and that the wall part of the body part is formed to have a set thickness so as not to give a varied volume. These demands are of course demanded in integral-type hollow ceramic components as well, so that a hollow ceramic component integrally having a body part and a narrow pipe part that can deal with these demands as well as a suitable method of producing the hollow ceramic component are demanded.
As conventional methods for producing a hollow ceramic molded article constituting a precursor of such an integral-type hollow ceramic component, there are the blow molding method disclosed in Japanese Laid-Open Patent Publication No. 10-81183/1998, the cast-molding method disclosed in Japanese Laid-Open Patent Publication No. 07-107333/1995, the reduced-pressure molding method disclosed in the aforementioned International Patent Application, and others. However, in any of these production methods, there are problems in producing an integral-type hollow ceramic component that can deal with each of the aforementioned demands and a hollow ceramic molded article constituting its precursor.
In other words, among these production methods, the blow molding method is a method in which a plastic pipe-shaped molded article is set between two molds of a forming mold, and a pressurized medium such as pressurized air is blown through an opening on one end of the molded article, so as to let the plastic molded article expand from the inside to the molding surface sides of the two molds. According to the blow molding method, a hollow ceramic molded article having an outer shape that conforms to the molding surfaces of the two molds of the forming mold is molded, and a hollow ceramic component can be obtained by baking this. However, by the blow molding method, it is difficult to control the inner shape of the ceramic molded article, so that the wall part of the body part will unlikely have a set thickness and also increases a possibility of lacking in uniformity.
Further, the cast-molding method is a method in which an aqueous slurry is injected through an opening of a casting mold so as to coat the molding surface of the casting mold with the slurry by utilizing the water absorption of the casting mold such as a plaster mold, and a superfluous slurry is discharged from the casting mold. In the cast-molding method as well, a hollow ceramic molded article having an outer shape that conforms to the molding surface of the casting mold can be molded. However, in the same manner as in the blow molding method, it is difficult to control the inner shape of the ceramic molded article, so that the wall part of the body part will unlikely have a set thickness and also increases a possibility of lacking in uniformity.
Furthermore, the reduced-pressure molding method is a method in which a plastic pipe-shaped molded article is set between the two molds of a forming mold and a negative pressure is imparted to a gap between the inner circumferences of the two molds and the outer circumference of the molded article so as to let the plastic molded article expand to the molding surface sides. By the reduced-pressure molding method as well, one can mold a hollow ceramic molded article having an outer shape that conforms to the molding surfaces of the two molds of the forming mold. However, in the same manner as the blow molding method and the cast-molding method, it is difficult to control the inner shape of the ceramic molded article, so that the wall part of the body part will unlikely have a set thickness and also increases a possibility of lacking in uniformity.